From Thursday through Monday, waves of water will wash over the Bay Area, drenching mountain tops in nearly a foot of rain, battering the Sierra with 100 mile per hour wind gusts, and triggering countless flash floods across the region.

The tool detects the radio frequencies that emit from a vehicle when someone inside is using a cellphone, according to the Virginain-Pilot newspaper. It’s not clear how the operator will be able to distinguish between a driver and a passenger in that same car texting.